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Abstract

Metal-insulator-silicon-insulator-metal (MISIM) waveguides are proposed and investigated theoretically. They are hybrid plasmonic waveguides, and light is highly confined to the insulator between the metal and silicon. As compared to previous ones, they are advantageous since they may be realized in a simple way by using current standard CMOS technology and their insulator is easily replaceable without affecting the metal and silicon. First, their structure and fabrication process are explained, both of which are compatible with standard CMOS technology. Then, the characteristics of the single MISIM waveguide whose insulator has its original or an adjusted refractive index are analyzed. The analysis demonstrates that its characteristics are comparable to those of previous hybrid plasmonic waveguides and that they are very effectively tuned by changing the refractive index of the insulator. Finally, the characteristics of the two coupled MISIM waveguides are analyzed. Through the analysis, it is obtained how close or far apart they are for efficient power transfer or low crosstalk. MISIM-waveguide-based devices may play an important role in connecting Si-based photonic and electronic circuits.

Figures (10)

(a) Cross-sectional structure of the MISIM waveguide. This structure is mainly analyzed in Section 3. (b) Cross-sectional structure of the MISIM waveguide whose insulator is partially removed by using wet-etching. The fabrication process of the MISIM waveguide consists of (c) formation of silicon patterns, (d) deposition of oxide, (e) formation of Si3N4 patterns, (f) deposition of metal, and (g) chemical-mechanical polishing. The cross-section along the line AB is shown in (h).

Intensity profiles of the MISIM waveguide mode for εM = εAu. For tI = 20, 40, 60, and 80 nm, they are shown in (a) to (d), respectively. (e) Intensity distributions along a horizontal line y = 125 nm for the different values of tI. An enlarged part is shown in the inset. (f) Intensity distributions along another horizontal line y = tI / 2 for the different values of tI. In (e) and (f), the black, red, green, and cyan lines correspond to the cases of tI = 20, 40, 60, and 80 nm, respectively. The correspondence between the line colors and the values of tI is used in the following figures.

Relations of the effective index neff to tI for different values of εM. The real and imaginary parts of neff, Re[neff] and Im[neff] are shown in (a) and (b), respectively. (b) also shows the relations of the propagation length Lp to tI, which are represented by the dashed lines. In the inset of (a), an enlarged part is shown. The square, circle, triangle, inverted triangle, and diamond symbols correspond to the cases of εM = εAu, εAg1, εAg2, εAl, and εCu, respectively. This correspondence between the symbol shapes and the values of εM is used in the following figures.

(a) Relations of the effective mode area Aeff to tI for the different values of εM. An enlarged part is shown in the inset. The dotted curve represents the real area AR of the thin insulator region R. In addition, the thin horizontal line represents the diffraction-limited area of silicon. (b) Relations of the normalized power in R to tI for the different values of εM. An enlarged part is shown in the upper middle inset. The lower right inset shows the definition of the thin insulator region R.

(a) Relations of the change of Re[neff], ΔRe[neff] to tI for the different values of εM. The dotted line represents the case of ΔRe[neff] = 0.001. The inset shows the inverted U shaped sub-region of the insulator, U, whose RI is nI + ΔnI. In this figure, ΔnI = 0.001. (b) Relations of ΔrRe[neff] to tI for the different values of εM. ΔrRe[neff] is the relative change of ΔRe[neff], which is given by (ΔRe[neff]/Re[neff])/(ΔnI/nI).

(a) Relations of the change of Im[neff], ΔIm[neff] to tI for the different values of εM when ΔnI = j0.00617, which means that the gain coefficient of U is 500 cm–1. The dotted line represents the case of ΔIm[neff] = 0.00617. Also, the relations of ΔrIm[neff] to tI are shown together. ΔrIm[neff] is the relative change of ΔIm[neff], which is given by (ΔIm[neff]/Re[neff])/(|ΔnI|/nI). (b) Relations of the increase of Lp, ΔLp to tI. Also, the gain coefficient of the MISIM waveguide mode for εM = εAg2 is shown as a function of tI.

(a) Relations of Re[neffd−neff]/Re[neff] to wM for the different values of tI. neffd denotes the effective index of the deteriorated MISIM waveguide mode of the structure in Fig. 1(h). (b) Relations of Im[neffd−neff]/Im[neff] to wM for the different values of tI.

(a) Cross-sectional structure of the coupled MISIM waveguides. (b) Relations of Re[Na] (black) and Re[Ns] (red) to sM. Na and Ns are the effective indexes of the antisymmetric and symmetric modes of the structure in (a). The upper (lower) insets show the distributions of the real part of the x component of the electric field of the antisymmetric (symmetric) mode, Re[Ex(a)] (Re[Ex(s)]) along the line y = 125 nm for sM = 60 and 300 nm, respectively. (c) Relations of Im[Na] (black) and Im[Ns] (red) to sM. In the calculation for this figure, εM = εAu, and tI = 40 nm.

Relations of the beating length Lb to sM (a) for the different values of tI and (b) for the different values of εM. In the calculation for (a), εM = εAu. In the calculation for (b), tI = 40 nm. Enlarged parts are shown in the insets.

(a) – (e) Relations of the coupling length Lc and the normalized, maximally-transferred power Pmax at Lc to sM for tI = 20, 40, 60, and 80 nm. In the calculation, εM was set to εAu for (a), εAg1 for (b), εAl for (c), εCu for (d), and εAg2 for (e). The correspondence between the line colors and the values of tI is the same as in the above figures. (f) Relations of Pmax in the case of sM = 60 nm to tI for the different values of εM. (g) Relations of the minimum values of sM to tI for the different values of εM. The correspondence between the symbol shapes and the values of εM is the same as in the above figures.